Background: Respiratory rate is a basic clinical measurement used for illness assessment. Errors in measuring respiratory rate are attributed to observer and equipment problems. Previous studies commonly report rate differences ranging from 2 to 6 breaths.min(-1) between observers. Methods: To study why repeated observations should vary so much, we conducted a virtual experiment, using continuous recordings of breathing from acutely ill patients. These records allowed each breathing cycle to be precisely timed. We made repeated random measures of respiratory rate using different sample durations of 30, 60 and 120 s. We express the variation in these repeated rate measurements for the different sample durations as the interquartile range of the values obtained for each subject. We predicted what values would be found if a single measure, taken from any patient, were repeated and inspected boundary values of 12, 20 or 25 breaths.min(-1), used by the UK National Early Warning Score, for possible mis-scoring. Results: When the sample duration was nominally 30 s, the mean interquartile range of repeated estimates was 3.4 breaths.min(-1). For the 60 s samples, the mean interquartile range was 3 breaths.min(-1), and for the 120 s samples it was 2.5 breaths.min(-1). Thus, repeat clinical counts of respiratory rate often differ by >3 breaths.min(-1). For 30 s samples, up to 40% of National Early Warning Scores could be misclassified. Conclusions: Early warning scores will be unreliable when short sample durations are used to measure respiratory rate. Precision improves with longer sample duration, but this may be impractical unless better measurement methods are used.
